1. Field of the Invention
This invention relates to a synthetic lubricating composition made up of a mixture of esters of particularly specified characteristics with regard to viscosity and a hydrogenated alpha olefin oligomer and to the lubrication of marine engines therewith.
2. Description of the Prior Art
The lubrication of marine engines, both of the spark ignition and diesel type, involves many unique problems. For the most part, such engines are designed for operation with mineral oils as lubricants. The metallurgy and other design features are selected for compatibility with oil fractions obtained by distilling from petroleum and finishing by solvent extraction, dewaxing and stabilizing by clay percolation or catalytic hydrogenation. For example, seals such as those at oil filter connections are fabricated of rubber formulation which will retain desired properties in contact with mineral oil fractions. These and other constraints on acceptability in service severely restrict freedom of choice in formulation of engine oils.
It has been known for some years that certain synthetic liquids afford properties that are superior to those of mineral oils. A considerable number of viscous esters have been proposed as lubricants, and some have attained commercial importance for such specialized purposes as lubrication of aircraft jet engines, hydraulic fluids and the like. These esters include: di-iso-nonyl dodecanedicate, nonanoic acid ester of trimethylolpropane, di-iso-tridecyl adipate, and the like.
Another class of known synthetic lubricants includes hydrogenated oligomers of 6-12 carbon atom alpha olefins as described in U.S. Pat. Nos. 3,382,291, 3,149,178 and 3,725,498. Of this group, particular preference is accorded hydrogenated oligomers of decene-1, octene-1 and mixtures thereof. The preferred molecular weight is the decene-1 polymer in which the trimer usually predominates. However, mixtures of octene-1 and decene-1 polymers also provide acceptable performance. Typical properties of a hydrogenated alpha decene oligomer and a mixed hydrogenated alpha decene/octene oligomer are shown in Table 1.
TABLE 1 ______________________________________ Alpha Decene Alpha Decene/Octene Oligomer (85/15 Wt. %) Oligomer ______________________________________ API Gravity 39.4 39.8 Kinematic Viscosity at 210.degree. F. cSt. 5.7 5.8 at 100.degree. F. cSt. 30 29 at -40.degree. F. cSt. 7,000 6,800 Viscosity Index 145 135 Pour Point, .degree.F. -65 -80 Flash Point, .degree.F. 450 440 Dimer Content, % 0.35 0.30 ______________________________________
Seal materials suited to these synthetic oils are available, but as previously noted, engine manufacturers must design their product for adaptability to mineral oils. As a result, the ester lubricants cause undue swelling of seals now used in automotive engines. The hydrogenated oligomers shrink such seals and permit leakage.
Blends of certain esters and oligomers have been marketed as engine oils, which blends behave in the same manner as mineral oils with respect to seals. These have been fully formulated oils including additives to impart antioxidant, antirust, dispersant and detergent properties as well as viscosity index (V.I.) improvement.
While they do not teach or suggest the lubricating oil blends of this invention, U.S. Pat. Nos. 3,763,244, 3,481,873, 3,843,535 and 3,860,522 are of interest. Other patents of interest are U.S. Pat. Nos. 3,694,382 and 4,010,106.